1. Field of the Invention
The present invention relates to a miniature motor and, more particularly, to a miniature motor that can be started easily.
2. Description of the Related Art
Conventional motors generally include a base, a rotor and a stator unit. The base includes a shaft tube to which the rotor is rotatably mounted. The stator unit is mounted to the base and comprised of a plurality of silicon steel plates and a plurality of coils. The rotor can be driven by a magnetic field created by the stator unit when supplied with electricity. However, difficulties and dead angles exist during starting of the motors. Easy-to-start designs for motors have been proposed to solve these problems.
FIG. 1 shows a conventional motor including a rotor 82 and a stator 8 having a plurality of stacked silicon steel plates 81. Each silicon steel plate 81 includes a plurality of radial extensions 811 each having a pole shoe 812 at a distal end thereof Each pole shoe 812 has two sections on opposite sides of a central line of the pole shoe 812. The sections are asymmetric to each other relative to the central line to provide a shift of magnetic flux. Thus, the stator 8 can create an asymmetric magnetic path to change the waveforms of the output torque while starting a rotor, eliminating dead angles of starting. An example of such a structure is disclosed in Taiwan Patent No. I221048. However, the asymmetric shape of the pole shoes 812 of the stator 8 renders a complicated structure as well as difficulties in manufacturing the silicon steel plates 81, increasing the manufacturing costs. Furthermore, the asymmetric pole shoes 812 increase the gap between the stator 8 and the rotor 82, adversely affecting the operational efficiency of the motor. Further, the stator 8 comprised of the silicon steel plates 81 has a certain axial height and a complicated structure, resulting in difficulties in designs of compact, miniature motors.
Taiwan Patent Publication No. 505337 discloses an easy-to-start brushless DC motor including a stator having upper and lower pole pieces each having magnetic plates angularly spaced at regular intervals. Taiwan Patent Publication No. 539346 discloses a set of silicon steel plates forming first and second poles having a phase difference in a magnetic pole angle therebetween. Taiwan Patent Publication No. 555290 discloses a stator with a radial winding, with a magnetic pole face of each pole plate of the stator having a notch. Taiwan Patent No. I287909 discloses an easy-to-start motor including a stator unit having a magnetic pole face with a stronger magnetism section and a weaker magnetism section. The stators of these patents, though having different shapes, still require complicated manufacturing processes and high manufacturing costs. The resultant motor structure is complex and difficult to reduce in size due to certain heights of the stators.
FIG. 2 shows another conventional easy-to-start brushless DC motor including a base 91 and a rotor 92. The base 91 includes an annular wall to which a plurality of coils 911 and an IC controlling element are mounted. The annular wall includes a positioning hole 912 between two adjacent coils 911. A positioning member 913 is received in the positioning hole 912. The rotor 92 is rotatably mounted to the base 91. The rotor 92 includes an annular permanent magnet 921 having a strong magnetism section. When the rotor 92 stops rotating, the positioning member 913 is magnetically attracted and positioned by the strong magnetism section, assuring starting of the motor by avoiding failing of detection of the polarity of the permanent magnet 921 by the IC controlling element. An example of such a structure is disclosed in Taiwan Patent Publication No. 523219. Although the motor of FIG. 2 without stacked silicon steel plates can be manufactured by simpler processes, the base 91 must include the positioning hole 912 to receive the positioning member 913 for assuring reliable starting of the motor, leading to difficulties in manufacturing the base 91. Furthermore, the positioning member 913 must extend a certain length along an axial direction of the rotor 92 so as to align with the coil unit 911 around the annular wall of the base 91, leading to difficulties in reducing the axial height of the base 91 having the positioning member 913 and in designs of compact, miniature motors.